Energy, Agriculture, and the Environment: Dead Zones and the Oil Spill in the Gulf of Mexico

Catastrophic, tragic, disastrous: these are all words that have been used to describe the BP oil spill in the Gulf of Mexico. It is impossible to deny that these words apply – thick, goopy crude has already coated the beaches and estuaries of the Gulf, contaminating more than 120 miles of coastline. The spill is an unprecedented environmental and human disaster that is posing significant challenges to an already-beleaguered tapestry of ecosystems in the Gulf. It becomes even more harrowing to consider that the BP spill has occurred at a time of year exactly when agricultural runoffs begin to flow into the Gulf from the Mississippi river, exacerbating the infamously-named Dead Zone.

Dead zones occur when runoff of nitrogen and phosphorus from agricultural fertilizers give rise to massive blooms of microscopic algae, which die and sink to the sea floor. Bacteria feast upon these dead algae, consuming oxygen in the water and reducing overall oxygen concentrations. Areas in which this occurs – and are not limited only to the Gulf – are known as either hypoxic (little oxygen) or anoxic (no oxygen). Anoxic systems have no oxygen present in the water; hypoxic systems have oxygen saturation in the range of 1-30%. Since fish and other marine biota cannot live below 30% saturation, both hypoxic and anoxic areas are termed “dead zones.”

The dead zone in the Gulf of Mexico begins to form each April, when the growing season begins in the Midwest, and peaks in July. In 2002, the hypoxic area reached the size of the state of Massachusetts. While the hypoxic zone varies year-to-year in its size, shape, and depth, it always remains below the biologically-rich mixed-layer of the ocean, the topmost layer that is churned and aerated through wind-driven ocean currents. It is this oxygen-rich surface environment, however, that is being affected by the BP oil spill.

The effects of oil on the ocean surface layer, also known as the mixed layer, are multiple. For one, oil forms a barrier that blocks the exchange of oxygen and CO2 into and out of the mixed layer, although choppy waters can limit the severity of this effect. But choppy waters are a double-edged sword as the churning of the waves creates a poisonous oil-and-water “mousse” emulsion, thus reducing food sources for seabirds and other wildlife.

In addition to the barrier effect, carbon-rich oil could be leading to a population explosion of the microbes that thrive around natural undersea oil seeps – the same microbes that flourish in nitrogen-rich waters. Just as microbes deplete oxygen as they work at a feverish pace to break down nitrogen from runoff , it is likely that microbes breaking down oil particles in the Gulf are also consuming oxygen. Indeed, as early as May 15, scientists were concerned that the very presence of oil in the waters of the Gulf – particularly the submerged and gigantic plumes – would deplete oxygen concentrations. According to researchers from the University of Georgia, it had already been observed that oxygen levels near the plumes were 30% lower than normal. “At the moment, we are seeing some indication that the oil spill is enhancing hypoxia,” said professor Nathaniel Ostrom of Michigan State University and Gulf specialist. “It’s just another insult to the ecosystem – people have been worried about the size of the hypoxic zone for many years.”

In addition to the oil itself, the chemical dispersants used to decrease the concentration of oil in the water have detrimental effects that further exacerbate the hypoxia. The oil poisons the lessvisible plankton, sand crabs, and fish larvae that form the foundation of the food chain in the Gulf. While nutrient-rich waters initially lead to an intense flourishing of microbes (and the hypoxic depletion of oxygen), toxic oil dispersants will perhaps have the most detrimental long-term effects on the basic bacteria and infauna that form the foundation of the food chain, and thus marine ecosystem. The chemical dispersant Corexit-9500 has drawn a significant amount of scrutiny in the BP oil spill tragedy. Developed after the Ixtoc I spill off Mexico in 1979, Corexit is produced by a subsidiary of Exxon-Mobil and is roughly four times more toxic than oil itself.

Not only is Corexit 9500 itself toxic, its main purpose is to dissolve thick oil into smaller particles. These are more easily consumed by already-present microbes in the water, with significant repercussions for the Gulf food chain. Thomas Shirley, a marine biologist at Texas A&M noted in an interview with Nature that toxic compounds from oil floating in plumes – huge underwater concentrations of oil likely created through the use of dispersants – could threaten species near the surface like fish, and that toxins from the plume could affect deep corals and other species. The article goes on to summarize that “deep-dwelling organisms such as zooplankton might be hit by the low oxygen levels in the plume… [also blocking] the normal up-and-down daily migration of numerous organisms [as well as] the flow of particles of organic debris from the surface to the deep where they are a critical food source.” Indeed, the EPA’s directive to cease the use of the highly-toxic dispersant reflects the severity of those repercussions. But as BP has repeatedly and categorically flaunted that regulatory order, it is an especially somber landmark that BP officially pumped its one-millionth gallon of Corexit into the Gulf on June 4, and continues to be pumped directly into the gushing oil at the top of the wellhead.

What is additionally troubling about the convergence of the BP spill and the dead zone is that initially it was estimated that the two would not converge; that is, the extent of the BP spill was estimated not to become so large that the two could intersect. The hypoxic zone in the Gulf typically extends from the mouth of the Delta to the west. The oil spill, conversely, has been carried by wind currents to the east. But as information and video about the flow rate at the well head has been released by BP, it seems that even initial worst case scenarios are greater than anyone imagined. The oil spill is, in essence, overlapping with and extending the annual hypoxic zone far, far to the east.

It’s also disturbing to consider how the worst-case flow rate estimate has evolved over time – BP‘s initial estimates were at 1,000 barrels per day; NOAA then estimated 5,000 barrels per day based on satellite imagery. When the underwater plumes of oil were discovered (and, for a time, denied by BP to exist) it was made clear that satellite imagery would not be an appropriate tool to measure the magnitude of the disaster. But even 5,000 barrels per day obscures NOAA’s initial estimates. According to video captured by a NOAA cameraman on the day of the explosion, NOAA scientists placed the flowrate somewhere between 64,000 and 110,000 barrels per day – the equivalent of three Exxon Valdez spills flowing into the Gulf of Mexico every week. Recent estimates based on BP‘s instrumentation and live video feeds suggest that the well is leaking a staggering 2,500,000 US gallons per day. To date, the spill has released an estimated 260,000,000 gallons of oil which government scientists have modelled as travelling as far as the mid-Atlantic ocean.

And what of the hypoxic zone? With resources and attention diverted to the spill, there is has been little or no press concerning the effects of runoff into the Gulf this year. It can’t be said that this is without good reason – the spill is on a magnitude far greater and its destruction potentially far more reaching than even the most severe dead zone, recorded in July 2002.

The combined effects of the spill and the annual hypoxic zone are numerous – nitrogen and carbon concentrations lead to the flourishing of oxygen-consuming bacteria; the oil barrier and diminished bacteria populations will detrimentally affect the uptake of CO2 and oxygen into the oceans; toxic oil and dispersants will negatively affect populations for decades to come.

The spill is, ultimately, a criminal tragedy – a catastrophe of unprecedented scale, the terribleness of which words can never appropriately convey. There is no real silver lining, despite Tony Hayward’s entreaties that the spill “will ultimately be beneficial to the Gulf in the long-run” or President Obama’s insistence that “we will leave the Gulf in better shape than it was before.” Coupled with the regular hypoxic event, it becomes apparent that the Gulf is in trouble, and that present energy practices – including the use of oil for agriculture – will continue to pose serious risks and challenges to the beautiful and fragile ecology of the Gulf of Mexico.

2009 hypoxia data from NOAA hypoxia watch

NY times graphic demonstrating the overlap of the hypoxic zone and the oil slick, as of May 8, 2010.

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